1. Field of the Invention
The present invention relates to a recording/reproducing apparatus for recording transmitted information signals, such as digital picture data, on a recording medium in a high-efficiency coded form and reproducing the original information signals by decoding high-efficiency coded signals read from the magnetic medium. More specifically, the present invention relates to a recording/reproducing apparatus which prevents data deterioration due to the high-efficiency coding at a time of double recording (hereinafter referred to as dubbing) for recording information signals reproduced from a recording medium to another recording medium.
2. Description of the Related Art
As is well known, the standardization of techniques of high-efficiency coding of digital data is being pushed actively. At present there are three standards: Recommendation H261 for television conference/television telephone; JPEG (Joint Picture Experts Group) for color still pictures; and MPEG (Moving Picture Experts Group) for storage media such as CD (Compact Disc)--ROM (Read Only Memory) (NIKKEI ELECTRONICS, 1990. 10. 15 (No. 511) P.124-P.129). On the other hand, digital recording/reproducing apparatus is also being developed which records information signals on recording medium in a high-efficiency coded form and decoding high-efficiency coded information signals read from the recording medium to thereby reproduce original information signals. As such a digital recording/reproducing apparatus, video cassette recorders (VCRs) using magnetic tape as recording medium and IC (Integrated Circuit) memory devices using semiconductor memories as recording media have been developed.
At present the leading high-efficiency coding technique uses orthogonal transformation which is referred to as DCT (Discrete Cosine Transform).
FIG. 1 illustrates a prior art high-efficiency coding system using the DCT technique. In FIG. 1, an input terminal 11 is supplied with a digital picture signal as an effective transmission information signal. The digital picture data is transmitted on a field-sequential basis in the NTSC system. Thus, a successive two fields stored in a frame memory 12 produce one frame of digital picture signal. In this case, the digital picture data is produced by encoding a luminance signal Y, a red signal component Cr and a blue signal component Cb. Unless otherwise specified, the luminance signal Y is taken as an example in the following description.
When one frame of digital picture data is sampled at, for example, 4 fsc (14.3 MHz), the number of pixels on one horizontal line is 910 because there are 910 samples in the horizontal direction. There are 525 lines in the vertical direction and thus the number of pixels in the vertical direction is 525. That is, the total number of pixels at the time of sampling is 910.times.525. However, only about 80% of the total number of pixels (768 pixels in the horizontal direction.times.488 pixels in the vertical direction) can be watched on the screen as effective pixels. The effective pixels form digital picture data applied to the input terminal as an effective transmit information signal. One frame of digital picture data stored in the frame memory 12 is read in blocks of four pixels (horizontal).times.four pixels (vertical) and applied to a DCT circuit 13 where the digital data is subjected to an orthogonal transformation process on a block-by-block basis.
The orthogonal transformation transforms digital picture data from time axis to frequency axis on a block basis and produces low frequency components and high frequency components in the order of increasing frequency two-dimensionally in the horizontal and vertical directions. The data is arranged such that it changes from direct current through low frequency to high frequency as an arrow indicating a zigzag scan shown in FIG. 2 advances in the horizontal and vertical directions. The data subjected to the orthogonal transformation is delayed by a frame delay circuit 14 by a one-frame period of time corresponding to a calculating time of an activity calculation circuit 22 described later and then applied to a scan conversion circuit 15.
The scan conversion circuit 15 scans data of a block in such a zigzag manner as indicated by an arrow of FIG. 2 on the basis of the contents recorded in a standard scan table 16 and rearranges them one-dimensionally so that direct current components and high frequency components can be output in sequence in the order of increasing frequency in the horizontal and vertical directions. This is because, from the standpoint of reproduction of an original image when the bit rate is decreased, sequential transmission of a direct current component and a high frequency component in the order of increasing frequency can reproduce a visually good image at a lower bit rate and, hence, with higher efficiency. The data scanned and converted in such a manner is generally larger in data amount than the original digital image data. Thus, data compression is not achieved without modification. For this reason, a quantization circuit 17 is used for requantization.
The quantization circuit 17 decreases data amount by dividing the scan-converted data by the result of multiplication of the contents recorded in a basic quantization table 18 and a coefficient a, which will be described later, by a multiplication circuit 19. The data requantized by the quantization circuit 17 is further applied to a variable-length coding circuit 20 for efficient transmission coding. The coding technique, which is most used by the variable-length coding circuit 20 is the Huffman coding or Run length coding, in which the number of successive 0s and the number of the following digits other than 0 in a requantized output are combined to allocate fewer bits in the order of decreasing probability of its occurrence. The number of bits is two at a minimum and several tens at a maximum. Thus, the data compression is performed on the digital image data. The data subjected to the data compression is taken from an output terminal 21 and then recorded on a recording medium (not shown).
In order to compress data while keeping picture quality, the requantizing process by the quantization circuit 17 is the most important. The performance of the requantization depends on calculation of the coefficient a, by which the basic quantization table 18 is multiplied, according to the basic quantization table 18 and the input digital image data. The picture definition (a rate at which fine detail and high frequency components are contained in a picture) is used for the calculation. That is, the coefficient a is calculated by the activity calculation circuit 22 using, as a measure of evaluation, a standard deviation or a quantity extracted from high frequency components output from the DCT circuit 13. The result of this calculation is converted to the coefficient a by a coefficient conversion circuit 23, which is in turn applied to a multiplication circuit 19.
At the time of reproducing of the high-efficiency coded data from the recording medium, the variable-length coded data is read from the recording medium and then subjected to processes which are the inverse of those at the time of the coding of data, i.e., inverse quantization, inverse scan conversion and inverse DCT processing. Thereby, the original digital picture data is recovered and displayed as a picture.
However, the conventional high-efficiency coding system using the DCT technique described above suffers from the following problems. Suppose now that an original picture signal is recorded on a first recording medium in the high-efficiency coded form. Further suppose that such outputs of the scan conversion circuit 15 as shown in FIG. 3A are applied to the quantization circuit 17, such contents as shown in FIG. 3B are recorded in the basic quantization table 18 and the coefficient a at this time is 2. Then, the quantization circuit 17, which multiplies each of the values of the basic quantization circuit 18 by the coefficient "2" and divides a corresponding input value of the quantization circuit 17 by each of the multiplication results. As shown in FIG. 3C, therefore, each output of the quantization circuit 17 is decreased in quantity of data. The data compressed as shown in FIG. 3C is subjected to the variable-length coding process and then recorded on the first recording medium.
At the time of reproducing of the first recording medium, expansion of data is performed by the inverse quantization processing of multiplying each input value of the basic quantization table 18 by the coefficient "2" used at the time of the compression and multiplying each value compressed as shown in FIG. 3C by a corresponding one of the multiplication results. In this case, as shown in FIG. 3D, of the results of the inverse quantization, the portions marked with .times.have values smaller than corresponding original values shown in FIG. 3A. That is, data deterioration occurs in those portions. However, with the high-efficiency coding process using the DCT technique, such a degree of data deterioration is inevitable because it is an irreversible coding system and is not a problem to be solved by the present invention (at this time, truncating process is used for the calculation, discarding the decimal fractions).
A problem to be solved by the present invention arises when data is reproduced from the first recording medium and then recorded on a second recording medium, namely, at a time of dubbing. Suppose now that the outputs of FIG. 3D obtained by inverse quantization of the reproduced data from the first recording medium are recorded on the second recording medium. Suppose that, in the apparatus for recording data on the second recording medium, the coefficient a is calculated on the basis of the reduced data shown in FIG. 3D to be "1.8" which is 10% smaller than "2". Then, the recording apparatus multiplies each value of the basic quantization table 18 shown in FIG. 3B by the coefficient "1.8" and divides each value of FIG. 3D by a corresponding one of the multiplication results. As a result, values shown in FIG. 4A are obtained. The data compressed as shown in FIG. 4A is recorded variable-length coded on the second recording medium.
At the time of reproducing of the second recording medium, the apparatus performs data expansion by the inverse quantizing process of multiplying each value of the basic quantization table by the coefficient "1.8" used at the time of the data compression and multiplying each of the values compressed as shown in FIG. 4A by a corresponding one of the multiplication results. FIG. 4B shows the results of the inverse quantization. All the values are smaller than corresponding values of FIG. 3D and data is deteriorated considerably. If data reproduced from the second recording medium is dubbed onto a third recording medium, more serious data deterioration will occur.
That is, a problem arises in the case of dubbing digital data reproduced from a recording medium onto another recording medium using the high-efficiency coding and decoding system in that the more the number of times of the dubbing increases, the greater is the data deterioration due to the use of the irreversible high-efficiency coding and decoding system.